Fix git.index imports

- Add the base, fun, typ, and util Python submodules of git.index
  to git.index.__all__ to restore the old behavior.

- Import them explicitly for clarity, even though they are bound to
  the same-named attributes of git.index either way. This should
  help human readers know what the entries in __all__ are referring
  to, and may also help some automated tools.

  This is a preliminary decision and not necessarily the one that
  will ultimately be taken in this import refactoring. It *may*
  cause some kinds of mistakes, such as those arising from
  ill-advised use of wildcard imports in production code *outside*
  GitPython, somewhat worse in their effects.

- Remove the ab.diff file that showed the problem this solves.

This resolves the problem deliberately introduced in the previous
incomplete commit, which modattrs.py output and test_imports test
results confirm.

git/index/__init__.py

@@ -4,6 +4,10 @@
 """Initialize the index package."""
 
 __all__ = [
+    "base",
+    "fun",
+    "typ",
+    "util",
     "BaseIndexEntry",
     "BlobFilter",
     "CheckoutError",
@@ -12,5 +16,6 @@
     "StageType",
 ]
 
+from . import base, fun, typ, util
 from .base import CheckoutError, IndexFile
 from .typ import BaseIndexEntry, BlobFilter, IndexEntry, StageType